because a "marriage" -- by definition -- is between a man and a woman. Your question is no different than asking why males cannot conceive children. If you guys want to shack up and play house, that's legal. If you want to form a "union," that's legal. But wanting to call it a "marriage" when two people of the same sex get together is a contradiction to the term itself. The only reason homosexuals want to call what they're doing "marriage" is so their sexual preferences will seem more legitimate.
But homosexuality will never be legitimate. It will always be viewed as freaky when one guy sticks his sexual appendage into another guy's mouth or rectum. It will always seem weird when a guy wears girls' clothing, sways like a Paris Hilton when he walks, or talks like a valley girl. These things will always make a large segment of the population laugh, cringe or simply be revolted. Why should we change the law -- not to mention the definition of a word -- just to make one small group of people feel better about themselves for their freaky actions?